1. Field
The present invention relates to a process for producing a metallic-nanoparticle inorganic composite constituted of a transparent oxide, e.g., SiO2, as a matrix and nanoparticles of a metal, e.g., silver, dispersed in the matrix at a high density. The invention further relates to a metallic-nanoparticle inorganic composite produced by this process.
2. Description of the Related Art
Surface plasmon is an electromagnetic mode having the property of causing electric-field localization and electric-field enhancement, and investigations on diversified applications thereof are recently being made in the fields of nanotechnology and biotechnology. One of the applications of surface plasmon is plasmon polariton in which light is transmitted with metallic nanoparticles having a size on the order of nanometer. For use in such applications, various processes for producing metallic nanoparticles are being investigated. Furthermore, investigations are being made also on the production of a two-dimensional or three-dimensional structure in place of a one-dimensional structure, in order to enhance signal intensity.
The most common process for producing a metallic-nanoparticle structure in this field is electron beam lithography. In this technique, the high-degree CMOS technology and an expensive apparatus are essential. In addition, it is basically difficult to produce a three-dimensional structure.
Besides the production processes employing electron beam lithography, the production of metallic nanoparticles based on chemical synthesis are being enthusiastically made in order to produce metallic nanoparticles at lower cost. Examples of such chemical processes are described in: a report that in the Zsigmoddy method, which is one of reduction processes, the pH of a solution was rapidly changed in a reduction step to evenly form a silver film of 10-20 nm on the surface of fine silica particles (80-180 nm) and thereby produce silver nanoshells; a report that silver particles were added to an aqueous gold chloride solution and used as a reducing agent to form gold shells on the surface of silver nanoparticles; and a report on silica spheres having silver nanoparticles evenly dispersed on the surface thereof using tin chloride as a reducing agent (see Y. Kobayashi, V. S.-Maceira and L. M. L.-Marian, “Deposition of Silver Nanoparticles on Silica Sphereres by Pretreatment Steps in Electoroless Plating,” Chem. Mater., (2001), 13, pp. 1630-1633).
Although those chemical techniques can produce metallic nanoparticles, the sample is always obtained as a colloidal solution. In order fox the colloidal solution to be used as a device, it must be solidified. However, in most of the research works, no investigation has been made on a method for dispersing the nanoparticles in a transparent matrix while preventing the nanoparticles from aggregating.
There is a technique intermediate between the inexpensive chemical synthesis methods described above and the electron beam lithography having high precision. This a technique for producing an SiO2 film having gold nanoparticles dispersed therein, for example, by the co-sputtering of a metal, e.g., gold, and SiO2. This technique can produce a three-dimensional structure containing gold nanoparticles dispersed therein. In general, many research works are directed to a nonlinear optical material employing the local surface plasmon of, e.g., gold. Although a large amount of gold nanoparticles can be incorporated in three-dimensional arrangement into a transparent SiO2 film, a heat treatment at 500° C. or higher frequently necessary after film deposition because more sputtering results in gold nanoparticles which have poor crystallinity and are not spherical (see B. Zhang, H. Masumoto, Y. Someno and T. Goto, “Optical Properties of Au/SiO2 Nano-Composite Films Prepared by Induction-Coil-Coupled Plasma Sputtering,” Mater. Trans., 44[2], (2003), pp. 215-219). Furthermore, the necessity of a heat treatment makes it difficult to obtain evenness of particle diameter. In addition, since a particle diameter increase due to aggregation during heating is unavoidable, it has been difficult to reduce the diameter of metallic nanoparticles to 20 nm or smaller.
As explained above, the production process based on electron beam lithography has had a problem that the cost of production is high. On the other hand, with respect to the conventional chemical synthesis methods, no technique for solidifying metallic nanoparticles has been obtained. Furthermore, in the co-sputtering method, it has been difficult to realize size reduction in metallic nanoparticles and even distribution thereof in a matrix.